The Nanotube Computer
In the hype-filled world of nanotechnology, Phaedon Avouris, head of IBM Research’s nanoscience and technology group, has a reputation as a meticulous and somewhat skeptical scientist. By his own description, he is one of those researchers whom reporters call to get a “realistic assessment” of the latest nanotech breakthrough. These days, though, the IBM chemist sounds uncharacteristically upbeat.
The reason for his excitement can be seen in a microscopic image recently produced in his lab. It shows a thin thread draped over several thick gold electrodes. What is not so apparent is that the thread, a single carbon nanotube, has been modified and positioned so that it forms two types of transistors, each a few nanometers (billionths of a meter) in diameter- a hundred times smaller than the transistors now found on computer chips. What’s more, the nanotube transistors work together as a logic gate, the fundamental computer component responsible for selectively routing electrical signals, transforming them into meaningful ones and zeroes.
The IBM device is one of the first examples of electronic circuitry constructed out of individual molecules. And while it’s merely a crude laboratory demonstration, its successful fabrication is nevertheless a further tantalizing clue that carbon nanotubes could one day replace silicon crystals as the building blocks for ultrafast, ultrasmall computers. More measurements are needed, says Avouris, “but our current results show, after taking into account difference in size, nanotube transistors show a performance superior to that of state-of-the-art silicon transistors.”
Indeed, carbon nanotubes are, in theory at least, the ideal material for building tomorrow’s nanoelectronics. And now, a little more than 10 years after their discovery, nanotubes seem ready to make the transition from exotic laboratory wonders to materials useful in actual technologies. Prototypes of nanotube devices are being tested in everything from full-color flat-panel TV screens to ultrabright outdoor lighting to a simpler, smaller x-ray machine; consumers could be shopping for a flat-screen TV that uses nanotubes as early as Christmas 2003.
But it is in computer memory and logic that nanotubes could have their greatest impact. Microelectronics now use silicon transistors with features as small as 130 nanometers across, which means that Intel can squeeze some 42 million of these transistors onto its Pentium 4 chip. However, it’s getting harder-and far more expensive-to continue to shrink silicon devices. Using nanotubes or related materials called nanowires as tiny electronic switches would allow computer designers to cram billions of devices onto a chip. If these molecular transistors work-and that is still a big if-replacing silicon will likely take years. But the ambition, says Charles Lieber, a Harvard University chemist, is to build electronics with performance “orders of magnitude beyond silicon. We’re trying to break with what is being done, to really change things.”
Carbon nanotubes are sometimes described as, basically, soot. In fact, they can be found among the deposits formed when electricity arcs between two carbon electrodes. But describing nanotubes as soot is like saying diamonds are nothing more than compressed coal. Each carbon atom in a nanotube is naturally positioned in a chicken-wire lattice that wraps into a hollow pipe. This molecular perfection gives nanotubes their long list of unusual-and potentially useful-properties.
Knowledge of the carbon structure dates back to 1985, when researchers at Rice University in Houston discovered soccer-ball-shaped carbon molecules called fullerenes. Following the discovery, theoretical physicists predicted that tubular versions of this same carbon structure could exist and that such molecules would have a number of enticing properties, such as excellent electrical conductivity. Mildred Dresselhaus, a physicist at MIT, recalls calculating the likely properties of what she called carbon “nanotubules.” “We didn’t have them yet,” she says, but it was still possible to speculate on “what they might be like.”
Spurred by the growing excitement over the new form of carbon, Sumio Iijima, a physicist at NEC Research in Tsukuba, Japan, went hunting for carbon nanotubes in late 1990. Trained in electron microscopy, Iijima says he was used to “looking at all kinds of graphite and small diamonds.” Iijima also says he was “quite lucky” in being experienced in observing needlelike microscopic shapes; his PhD had been on microscopic whiskers of silver. Several months after beginning his search, Iijima hit pay dirt. “When I saw all these needles of carbon, immediately I came to the right answer,” he remembers.
What Iijima was peering at were “multiwall” nanotubes-long carbon molecules stuffed one within another like nested Russian Matryoshka dolls. In 1993, Iijima and his NEC coworkers, and another group at IBM Research in San Jose, CA, separately produced an even more exquisite version: nanotubes whose walls were only a single atom thick.
The new structures didn’t disappoint. One early research finding was that in the presence of an electric field nanotubes emit electrons from their extremely fine tips. Any number of electrically conductive materials will, when a high enough voltage is applied, spit out electrons. Nanotubes can do this at remarkably low voltages because of their extreme sharpness. So carbon nanotubes are almost perfect for building tiny, efficient electron emitters. They can direct focused electron beams at very small targets-say, a pixel of a display.
As many as two dozen electronics firms, including Samsung and Motorola, are now racing to develop flat-panel displays that use nanotubes. TV screens and the computer displays that sit on most desktops are holdovers of the vacuum-tube era. These clear and relatively cheap displays use cathode-ray tubes, in which electrically heated wires shoot electron beams onto a phosphor-coated screen, which in turn lights up. The problem is that the picture tube uses a lot of power, and it must be deep enough to allow the electron guns to project to the whole screen-hence the fat bulge in back of most TVs. In contrast, screens using an array of nanotubes can put tiny electron emitters behind each pixel and therefore can be far thinner.
At first glance, the prototype 13-centimeter screen made at the Samsung Advanced Institute of Technology in Suwon, South Korea, doesn’t look much different than any other small TV. Smiling actors flash across its face in a slickly made promo. But that similarity is exactly the point. If Samsung researchers can turn this prototype-which uses nanotubes to bombard the phosphor screen with electrons-into a TV as bright and clear as the one in your living room, they could capture the best of both display worlds: cheap as cathode-ray tubes and thin as far more expensive liquid-crystal or plasma display TVs.
Samsung expects to have full-color prototypes capable of the resolution needed for high-definition television this winter and an 81-centimeter TV ready for the market by late 2003 or early 2004, says Jong Min Kim, vice president of research at the Samsung Advanced Institute of Technology. Key to success in the $100 billion display market, he says, will be getting manufacturing costs of the nanotube TVs low enough that they can compete with cathode-ray-tube models. “First we will try to attack the TV market, then we’ll go after the computers,” says Kim.
Far from the large corporate labs working on nanotube TVs is a tiny Woburn, MA-based startup called Nantero, whose employees hope to take on another multibillion-dollar market-computer memory. Sitting in Nantero’s conference room, which also serves as a front entrance, lobby and kitchenette, cofounder and chief scientist Tom Rueckes seems both anxious and excited. And well he should be. The year-old company is promising a high-density nanotube-based memory that would revolutionize the market. And it claims it will have this breakthrough working within two years. “Imagine,” says Rueckes, “having several gigabits of memory at your fingertips that is instantly on.”
Indeed, the most attractive aspect of the Nantero memory is that it will be “nonvolatile.” Conventional dynamic random-access memory (DRAM), the short-term electronic memory that a computer uses to run its operating systems and programs, holds information only as long as the power is on. That’s why a PC needs to be booted up: the machine has to rewrite stored information from the hard drive onto the electronic memory. Nonvolatile memory means never booting up again. Eventually, if the storage capacity of nonvolatile memory chips gets large enough, they could make magnetic hard drives obsolete.The best existing DRAM can hold about one gigabyte of data. Within two years, Nantero expects to have a nanotube-based nonvolatile memory chip with several gigabytes of capacity.
The nanotube memory is based on an ingenious, though strikingly simple, design that Rueckes came up with while a PhD student under Lieber at Harvard. An array of parallel nanotubes is suspended just a few nanometers above a perpendicular array lying on a substrate; each intersection of the cross-arrays represents a potential bit of memory. When an applied electrical force stretches a tube in the top array close enough to a lower tube, they physically bind and a current can flow between them; the switch is on and stays on even when the power is turned off. Because each bit of memory is so small, a centimeter-sized chip based on the design could have, in theory, terabits (a trillion bits) of nonvolatile memory.
The goal is to turn this laboratory design into real technology as quickly as possible. Rueckes declines to detail exactly what has been built so far, except to say that “components of it are working.” But he adds that the strategy is to integrate nanotube memory with conventional electronics. “We want to come up with a product that can be manufactured with existing technology,” he says.
Such nonvolatile memory would change how people use their computers, doing away with those tedious minutes spent booting up. But the real prize in nanoelectronics-the one that will make people truly forget about silicon-is the logic circuits that are the brains of computers. Moore’s Law, the oft-cited 1965 prediction by Intel cofounder Gordon Moore that the number of transistors on a chip would double every 18 months, has held for more than three and a half decades. But experts predict that within a decade or so, it may well be impossible to make silicon transistors small enough to continue to uphold Moore’s Law.
There is no shortage of technologies proposed to eventually replace silicon, from ways to use complex organic molecules as transistors to “quantum computing” (see “Beyond Silicon,” TR May/June 2000). But carbon nanotubes are emerging as a leading candidate. Not only are they the right size, with the right electronic properties, but their compatibility with existing semiconducting materials raises the prospect that, over the next decade, it may be possible to gradually integrate them with conventional silicon technology. That could give nanotubes the inside track, since most chip makers are no more anxious than Rueckes to overthrow existing manufacturing techniques.
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